1. Field of the Invention
This invention relates to a process for producing a novel polyacetal resin composition which is conveniently used as a material for such parts as clips, springs, gears, bearings, cams, levers and the like in electrical and electronical machines, office machines, automobiles, industrial machines, toys and/or sports goods, etc.
2. Prior Arts
Polyacetal has found wide usages as an engineering plastics excelling in mechanical properties, electrical properties and chemical properties such as chemical resistance. Its usages are limited, however, because of its inferior impact resistance compared to that of other engineering plastics. As a method to improve impact resistance, i.e., impact strength, of polyacetal, a means to blend a thermoplastic polyurethane elastomer with polyacetal has been proposed [cf. Japanese Patent Publications Nos. 24,583/1963 and 1,522/1964; U. K. Patent No. 1,017,244; Japanese KOKAI (Laid-Open) Patent Publications Nos. 145,243/1984 (corresponding to U.S. Pat. No. 4,978,725), 155,452/1984 (corresponding to European Patent No. 117,664), 155,453/1984 (corresponding to U.S. Pat. No. 4,804,716), 191,751/1984, 19,652/1986 (corresponding to U.S. Pat. No. 4,707,525), 56,553/1988 (corresponding to U.S. Pat. No. 4,780,498) and 280,758/88 (corresponding to European Patent No. 290,761)]. Some of those proposals have already been put to practice.
However, shaped articles formed by injection molding such compositions of polyacetal blended with thermoplastic polyurethane elastomers generally exhibit, while they do show improved impact resistance, reduced tensile strength and elongation at break at weld portions, providing one reason for easy breakage of the injection-molded articles by external forces. Hence improvements in that aspect are in demand as well as still greater improvement in impact resistance.
There are also other proposals for a method of polymerizing an isocyanate compound with long chain polyol in the presence of polyacetal [Japanese KOKAI (Laid-Open) Patent Nos. 182,328/1988 (corresponding to U.S. Pat. No. 4,946,906) and 196,649/1988 (corresponding to European Patent No. 277,630)]. Alloys of polyacetal and polyurethane elastomers proposed therein exhibit better impact strength compared to the compositions obtained by simply melt-mixing polyacetal and polyurethane elastomer.